1. Field of the Invention
This invention relates to a method and apparatus for doping impurities into a semiconductor substrate.
It is a common practice in semiconductor manufacture, for instance for the purpose of manufacturing integrated circuits, power-semiconductor components and the like, to perform the doping of a flat substrate which is made essentially of silicon, hereinafter referred to as "Si-substrate"--at least in part by a method which is generally known as diffusion or impurity diffusion.
Depending on the type and the intended use of the semiconductor to be manufactured, the diffusion method can be applied with different doping additives, different methods of localization or, respectively, the control of the doping, and also by means of different process sequences.
Examples for known doping additives are arsenic, antimony and phosphorus (donor or, respectively n-doping), or boron, gallium and aluminum (acceptor, respectively p-doping), which may be evaporated in their elemental or necessarily chemically bonded form, such as, for instance in their oxidized form, and be applied to the supporting carrier substrate.
Typical examples of steps for known methods for the localization of the doping are, on the one hand, covering, or masking of the Si-substrate in the areas not to be doped and, on the other hand, the subsequent locally selective removal of the previously flatly doped areas and, if needed, with a subsequent thermal treatment (drive-in-stage), for the purpose of diffusing the doping additive deeper into the Si-substrate in the remaining doped areas, and after the selective removal process.
Of these steps for local selective doping, the covering, i.e. masking, is as a rule the simpler step when compared with the subsequent removal process.
In addition, for the purpose of a local removal of the Si/doping additive-phase generated by the diffusion, the underlying layer, i.e. the layer which has been exposed by the subsequent removal of the Si/doping additive phase, must have predetermined properties which cannot always be obtained or which can be obtained with great difficulty only. Finally, the usually occurring groove structure caused during the subsequent removal of the Si/doping additive phase may be detrimental to an otherwise practically planar surface of the Si-substrate.
For the masking of a predetermined cohesive surface, or the masking of predetermined partial areas of a surface, which is advantageous for several reasons, of areas of the Si-substrate not to be doped, a masking layer for preventing or effectively delaying the access of the doping means to the part of the substrate lying underneath the masking layer, and considering the methods of the process, is a prerequisite. Inasmuch as the doping by diffusion takes place in the presence of high temperatures of typically above 500.degree. C., the prior masking layers which contain organic components, such as, for instance, the conventional photo-lacquer masks, in this case are unsuited in principle because of their thermal decompoundability.
One known and thermally stable masking layer for diffusion processes is made of silicon oxide, for instance in the form of a coating created by epitaxy and the vapor application of silicon in the presence of trace amounts of oxygen on a substrate of silicon, or else in the form of a fused-on, glass-like coating on the basis of silicon dioxide.
It is, however, a known fact (see, for instance B. R. M. Burger and P. P. Donovan, "Fundamentals of Silicon Integrated Device Technology", Prentice-Hall, U.S.A., 1967, Volume I, pages 153-155), that of all of the conventional p-doping additives only boron has a sufficiently low diffusion coefficient in silicon dioxide, to permit the use of an adhesive masking layer of silicon dioxide for the effective masking of the Si-substrate.
Gallium, on the other hand, diffuses through silicon dioxide about 400 times faster than through silicon, given typical diffusion temperatures in the range of 800.degree.-1300.degree. C.; similar values must be used in connection with aluminum, and it may be assumed that, given the temperature conditions for this process, that aluminum diffuses at least about 100 times faster through silicon dioxide than through silicon.
Whenever a Si-substrate with an adhesive mask coating of silicon dioxide is doped with gallium or aluminum in a diffusion process, this is done, as noted in the publication cited above, on the assumption that the diffusion effect of these doping additives underneath the silicon dioxide coating of the Si-substrate is identical to the diffusion effect obtained without the presence of an adhesive silicon dioxide coating.
This means that a masking layer of an adhering silicon dioxide on the Si-substrate during the diffusion for doping with aluminum or gallium might be useful for other, previous or subsequent, processes or semiconductor functions, respectively, or that it remain useful, but that in no manner is it suitable as an effective masking layer for the localized selective doping with aluminum or gallium.
Silicon nitride, in contrast to silicon dioxide, is impenetrable to doping additives such as aluminum to such an extent that it would be suitable as a masking coating for the localized selective doping of Si-substrates. The investment in needed processes and apparatus, however, to produce silicon nitride layers on silicon substrates is so high and the reproducibility is so uncertain that this particular method is hardly suitable, at least not for purposes of commercial production.
Given the present state of the art, masking coatings of silicon dioxide can be used in commercial semiconductor production only if the doping is performed with boron, but not with aluminum or gallium, since so far there are no maskings and coverings of record which are sufficiently impenetrable to aluminum or gallium and which can be applied at reasonable expense.
Aluminum as such is preferable as a doping additive for Si-substrates because it diffuses relatively quickly into silicon and since it is relatively easy to handle. For purposes of locally selective p-doping using the diffusion method, aluminum so far could be used only with the comparatively unfavorable method of subsequent removal of the aluminum-silicon phase created during the vapor application or diffusion, respectively, or by using the technically demanding silicon nitride coating for a masking of the Si-substrate.